The present invention relates to measuring the geometry of 3-dimensional objects, and particularly measuring their surfaces, and a method and an apparatus for improving the measurement accuracy of such methods.
The three-dimensional measurement of objects or surfaces of objects has a multiplicity of applications in industrial production. In particular, for inspecting production results, methods enabling a quick quality check are employed here, such as the shape measurement of motor vehicle brake discs or the measurement of motor vehicle tires with respect to a possible vertical or lateral run-out.
By means of three-dimensional geometrical measurement methods, moreover, mechanical vibrations of surfaces, for example, may be determined or the thickness of materials produced in several sheets, such as sheet metal or paper, can be measured. While some applications aim to determine the exact geometrical shape of the surface of an object, other methods rather aim at checking the compliance with a parameter in only one geometrical dimension. With smooth surfaces, for example, it is mostly of interest as to whether the surfaces have a roughness or ripple below a certain limit value. In general, for example, a smooth surface could even be defined by the fact that the lateral structuring of the surface is significantly less than the desired measurement accuracy in the direction of the surface normal.
The quick detection, measurement and checking of the geometry of industrially manufactured products is an important field of application of contactless 3D measurement systems, as already mentioned. Usually, such systems are digital, i.e. they use digitized measurement values from measuring elements in a finite resolution of digitization. Quantization errors may be caused, on the one hand, by the digitization of originally analog measurement values, or by per-se quantized detectors, such as CCD cameras.
Standard methods for measuring surfaces here are the tactile shape measurement working point by point by way of mechanical measurement probes scanningly guided across the surface of the object to be measured. Another high-resolution method is laser triangulation. The great disadvantage of these methods here consists in the fact that the surface of the object has to be scanned point by point, i.e. in a rastered fashion, which is extremely time-consuming. Due to the extremely high measurement speed achievable, the light-slit method is increasingly gaining importance in 3D shape detection. In the light-slit method, a measurement light strip or measurement strip is projected onto a surface of the object to be measured, and the measurement light strip at the same is recorded by a camera or a comparable detector. The topography of the object to be measured can be inferred from the location of the measurement light strip in the camera image if the geometry between the object, the camera and the measurement light projector is known. The advantage of this method, above all, is the simultaneous parallel detection of a complete height profile along a line. The fact that a line can be measured in a single measuring step opens up the possibility of completely scanning and measuring a complete object in a short time by simple translational or rotational movements of the object and/or the measurement system in connection with measuring steps in close temporal succession. Typical measuring times needed for the capture of the entire object here amount to seconds. The achievable measurement accuracy is less than in the laser triangulation methods or the time-consuming geometric detection by means of tactile measurement probes, particularly in the case of very expansive objects. This is due to the basic functioning of the light-slit principle. A laser line or light line projected onto the object is detected by a parallactically tilted measuring camera by means of an image sensor with a finite number of picture elements (pixels). By way of the detection and digitization of the geometrical information by means of the determination of the coordinates of the light line on the sensor chip, a resolution or height resolution of such a light-slit method, which is limited by the sensor resolution (finite pixel dimension), results in a direct way. Typical pixel numbers in one dimension here range from 1,000 to 2,000. Thus, if the entire geometrical area of the chip in one dimension is used for height measurement, which necessitates precise adjustment of the geometry, a digitization resolution of such a system ranges from 1:1,000 to 1:2,000 due to the finite pixel number on the chip. The physically available height measuring range thus is divided into 1,000 to 2,000 values. Thus, a measurement resolution of 0.1 mm is achieved in a basically geometrically possible height measurement range of, for example, 100 mm.
The exact geometrical dimensioning here usually is limited additionally by the devices used, such as the CCD used, or the optical elements used for mapping. In particular, in a system with a given optical mapping property, the adjustment of the height measurement range is not completely independent of the chosen lateral width of the scanned measurement area on the object. A square CCD with 1,000×1,000 pixels may serve as an example. In the shape detection of a typical flat object, such as the surface of a brake disc, a lateral measurement width of 50 mm is needed, for example. In addition, height measurement accuracy of a few micrometers is demanded. Due to the demanded width, however, the mapping optics is to be adjusted so that the CCD maps an area of 50 mm×50 mm if the object is observed perpendicularly. This leads to a height resolution of a maximum of about 50 μm. With brake discs, however, height measurement accuracy of few micrometers is demanded, which cannot be achieved directly with such a system due to the above considerations.
As already described, since the height resolution of light-slit measurement methods depends on the geometry and/or the relative orientation of the camera with respect to the surface of the object and to the light projection means, among other things, the height resolution can be increased by limiting the basically available height measurement range. This can be achieved by arranging the laser so that the laser fan beam is incident on the object surface in a very flat and brushing manner (for example at 80° with respect to the surface normal), and at the same time the measurement camera is positioned perpendicularly thereto rather in vertical direction. Due to the geometry, a slight change in height on the surface of the object to be examined thus leads to a strong change in the position of the projected light strip. However, such an arrangement also leads to the fact that the position of the light-slit on the surface greatly depends on the object height and thereby “migrates” laterally when the object height changes. The above-described configuration with a very flat angle of incidence of the laser beam also leads to a very small working distance between the measuring means and the object surface, which is often not desired or possible due to the spatial situation and the safety measures needed (minimum distance of the measurement technology from the object).
In general, it is to be noted that such strong limitation of the basically available height measuring range is not advantageous and desired for industrial applications since, thereby, there is hardly any more flexibility in the application on different surfaces to be measured.
So as to increase the measuring speed of light-slit measurement systems, typically a specialized sensor permitting the signal evaluation already on the sensor chip itself is employed. Usually, to this end a preferential direction is defined on the sensor, in which the mapping of the height information of the light strip is expected on the sensor, or the sensor is adjusted so that the mapping of the light measurement strip migrates in the sensor preferential direction on the sensor. The evaluation of the measurement data on the sensor is then implemented so that, per pixel column in a preferential direction, exactly one mapping location is defined, which corresponds to the coordinate of the brightest pixel in the accompanying column. Hence, the height resolution is automatically limited by the quantization of the spatial resolution of the sensor. So as to further increase statistical fluctuations (photonic statistics) and hence the height resolution, a plurality of detection threshold values for determination of the coordinates of the light line on the sensor chip can be used in such a chip, whereupon the final coordinate is determined by averaging the coordinates determined by means of the different threshold values. Such methods have recently become available in the latest generation of light-slit cameras, for example by the company Sick-IVP. However, this leads to the fact that the available measuring frequency is effectively decreased significantly. Moreover, only in some few cases does such a method lead to a real increase in height resolution.
In principle, the resolution on a sensor, independent of the fact as to whether it has discrete sensor elements or not, can be determined algorithmically by way of forming a center of gravity of the light distribution on the image sensor detected by the sensor. In technology, this method is also known as the COG (center-of-gravity) method. In principle, with such a method it is possible to increase the accuracy with which the image of the light-slit measurement strip can be detected on the sensor area almost arbitrarily. In reality, due to technical limitations, such as the finite dynamic range of individual CCD pixels, improvements in resolution are possible up to a maximum of a factor of 10. The computational operations for this typically are intensive in computation time, such as the adaptation of suitable parameterizations by means of a least-square fit. Hence, these mostly have to be executed on a downstream computer and/or dedicated hardware. Like the methods for increasing resolution discussed above, this leads, in general, to a clear reduction in the maximum measuring speed possible.
Moreover, for executing the COG method, it is a mandatory prerequisite that the measurement light strip illuminates several pixels on a pixel sensor in preferential direction, the needed minimum width of the projected laser light line on the object thus automatically decreasing the achievable spatial resolution on the surface of the object to be measured. More generally speaking, there are certain limiting conditions regarding the needed width of the projected laser light line on the object if the improvement of the resolution is to be achieved by means of the COG method by using a multiplicity of sensor pixels for the evaluation.
As already mentioned above, light-slit sensors generally have a multiplicity of individual measurement tracks or pixel columns providing geometrical measurement values at the same time. A combination of several such measurement tracks to an effective new (wider) measurement track or the combination of data captured successively in time by means of averaging, i.e. reduction of the lateral resolution, generally also only leads to improvement in the statistical behavior of the measurement, i.e. to reduction of the standard deviation of several successive measurements or to improvement in the reproducibility of the measurement, but not to improvement in the measurement accuracy. Such measures do not influence the real mapping location of the measurement light strip on the sensor surface, which causes quantization of the possible coordinates due to its constructional principle. Figuratively speaking, for example, it would be the same (wrong, since only roughly resolved) measurement value that would be averaged, which in turn would lead to a faulty measurement value. As illustrated above, there are a series of approaches trying to improve the spatial resolution and/or the height resolution of a 3D geometrical measurement system, but with all these entailing the disadvantage of significantly prolonging the measurement duration needed.